In fabricating semiconductor devices, various heat treatments such as an oxidation treatment, a diffusion process, a chemical vapor deposition (CVD) process or the like are performed. A vertical type heat treatment apparatus having a boat with a plurality of wafers vertically stacked therein is used in performing such heat treatments.
The wafers are first vertically stacked at a predetermined interval inside the boat, and the boat is then loaded into a reaction chamber. Thereafter, the vertically stacked wafers are heat-treated using a heating unit while supplying into the reaction chamber gas chosen depending on the nature of the heat treatment.
Conventionally, a boat used in such vertical type heat treatment apparatus includes, as shown in FIG. 7, a top plate 10, a bottom plate 11 and three supporting bars 12, wherein the three supporting bars 12 are supported by the top and the bottom plates 10 and 11. A plurality of grooves 13 are formed in each of the three supporting bars 12, so that a plurality of wafers 4 can be vertically stacked and maintained therein. Referring to FIG. 7, only the grooves 13 formed in top and bottom portions of the supporting bars 12 are illustrated and grooves in remaining portions thereof are omitted for simplicity.
Further, Japanese Patent Laid-Open Publication No. 1997-251961 discloses two types of wafer loading methods: one employing either inclined or curved grooves into which the wafer is directly inserted and the other employing a ring-shaped (annular) susceptor onto which the wafer is placed. Similarly disclosed in Japanese Patent Laid-Open Publication Nos. 1994-163440 and 1998-50626 are boats using ring-shaped holders.
Direct placing of wafers in the grooves formed in the supporting bars of the boat, as described in FIG. 8, provides minimal surface contact therebetween (three contact points in case of FIG. 8). Such minimal surface contact generates augmented deformation (due to the weight of the wafer itself) and thermal stresses in the wafers, producing defects, e.g., slip, therein.
Further, if the grooves have sharp edges contacting the heated wafers, such local contacts can easily produce scratches on the wafers softened by heat.
Moreover, the situation can be further aggravated by large surface roughness of the grooves if exist, which further produces scratches thereon. Even in the case where the grooves are provided with inclined or curved edges, such still cannot prevent the slip generation in the wafers, due to its inherent nature of minimized local contacts having small contact areas.
Regarding the prior art holders or susceptors supra, the prior art references lack in their detailed usage and operation, e.g., wafer loading and unloading processes. Therefore, elaborate wafer holders need be devised for their practical implementation in the field of semiconductor fabrication.